The present invention relates to processes for fabricating structural units from titanium alloys.
Pure titanium is relatively soft, weak and extremely ductile. Through additions of other elements, the base metal is converted to an engineering material having novel characteristics, including high strength and stiffness, corrosion resistance and usable ductility, coupled with low density.
Titanium is allotropic. In unalloyed titanium, up to about 785.degree. C., the atoms arrange themselves in a hexagonal close-packed crystal array called alpha phase. When titanium is heated above the transition temperature (beta-transus) of about 785.degree. C., the atoms rearrange into a body-centered cubic structure called beta phase. The addition of other elements to a titanium base will generally favor one or the other of the alpha or beta forms, and will increase or decrease the beta-transus temperature.
Titanium alloys are classified into three major groups depending on the phases present: alpha, beta, or a combination of the two, alpha+beta. The elements which favor (stabilize) the alpha phase are termed alpha stabilizers, those which favor the beta phase are termed beta stabilizers, and those which do not show a preference for either phase, but promote one or more desirable properties are termed neutral. The alpha stabilizers raise the beta transus temperature, i.e., the temperature at which the atoms rearrange from the alpha form to the beta form, and beta stabilizers lower the beta transus temperature. Alpha stabilizers include Al, 0, N and C. Beta stabilizers include Mo, V, Ta, Nb, Cr, Mn, Fe, Si, Co, Ni, Cu and H. Neutral stabilizers include Zr and Sn.
The so-called beta titanium alloys are, in general. metastable, i.e., within a certain range of beta stabilizer content, the beta matrix can be decomposed by heating the alloy to a temperature below the beta transus temperature. Such decomposition can result in formation of allotriomorphic alpha phase or an intimate eutectoid mixture of alpha and a compound. The beta stabilizers which exhibit the former type of reaction are called beta isomorphous stabilizers while those which provide the latter reaction are called beta eutectoid formers.
The metastable beta titanium alloys may be divided into two major groups, the rich metastable beta alloys and the lean metastable alloys. Broadly, the division of metastable beta titanium alloys is made as a result of processing and heat treatment practices: Lean metastable beta alloys retain the beta phase at room temperature only after relatively rapid cooling through the beta transus, such as by water quenching, while rich metastable beta alloys retain the beta phase at room temperature even after relatively slow cooling, such as air cooling.
Titanium alloys are widely used in aerospace applications due to the characteristics listed previously. Titanium alloys have been fabricated into useful shapes by forging, rolling, extrusion, drawing, casting and powder metallurgy. In recent years, a fabrication technique known as superplastic forming (SPF), with or without concurrent diffusion bonding (DB). has achieved a certain prominence. This process makes it possible to form titanium alloys in a simple manner with significant reduction in parts such as fasteners, thereby permitting the fabrication of airframe and engine structures with significant cost and weight reduction.
The production of SPF/DB components requires titanium alloy sheets and foils with uniform and fine grain structure. Such fine grain structure can require multi-step and expensive thermomechanical and/or thermochemical processing to convert ingot material into sheets and foils. In certain alloys, such as, for example, rich metastable beta alloys, a uniform fine grain structure is very difficult to obtain due to their high resistance to cold deformation which is the same in all locations. Such alloys may require expensive multiple rolling/annealing cycles to achieve thin sheets or foils with a desireable uniform fine grain structure.
Sandwich panels, such as lightweight core laminate panels, have the advantage over conventional construction materials that they combine a low weight per unit area with exceptional flexural rigidity and good vibration damping. Such panels conventionally consist of two relatively thin outer covering layers of a hard, firm and rigid material. These two covering layers are joined together by a relatively thick core which consists of a light and less rigid material. The bond between the core and the covering layers must be sufficiently strong that no detachment of the covering layer from the core occurs upon application of a force.
Accordingly, it is an object of the present invention to provide a process for producing SPF/DB components.
Other objects, aspects and advantages of the invention will be apparent to those skilled in the art from a reading of the following detailed disclosure.